You can recast MG before the last tick without losing the last tick of the one you are already channeling.

Actually, there's a clever work around for that, which is using yourself as target for command demon, in other words: /cast [@player] command demon
You can't silence yourself so in case of sacced felhunter, the macro will just give an error when it reaches that point, which is after healthstone/dark regen

I have one for UA, Corrruption, Agony, Fear, Mortal Coil, Banish(PvE), and Command Demon.

You might be better off turning on the Cast on Focus Modifier in the interface options. The lets you press shift, ctr, or alt and any keybind you press (that isn't already bound with one of those) will cast at your focus.